Typological study and statistical assessment of

Typological study and statistical assessment of parameters influencing earthquake vulnerability of commercial RCFMI buildings in New Zealand Rijalul Fikri, Dmytro Dizhur & Jason Ingham

Bulletin of Earthquake Engineering Official Publication of the European Association for Earthquake Engineering ISSN 1570-761X Bull Earthquake Eng DOI 10.1007/s10518-018-00523-x

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Author's personal copy Bulletin of Earthquake Engineering https://doi.org/10.1007/s10518-018-00523-x ORIGINAL RESEARCH

Typological study and statistical assessment of parameters influencing earthquake vulnerability of commercial RCFMI buildings in New Zealand Rijalul Fikri1 · Dmytro Dizhur1 · Jason Ingham1 Received: 22 May 2018 / Accepted: 16 November 2018 © Springer Nature B.V. 2018

Abstract Reinforced concrete frame with masonry infill (RCFMI) buildings comprise a significant proportion of commercial buildings constructed prior to the adoption of New Zealand’s modern seismic design codes in 1976. The characteristics and seismic performance of RCFMI buildings have not been previously investigated at a national level. As part of the study reported herein, efforts were made to identify and document building characteristics, including building address, infill type (clay-brick or concrete-block masonry infill), wall morphology (cavity or solid wall), geometry (building footprint and height), building continuity, and age of construction. During sidewalk surveys the characteristics of 203 and 55 RCFMI buildings were observed and well documented in the Auckland and Dunedin regions, respectively. The surveyed RCFMI buildings were assigned to one of four typologies according to infill type and wall morphology. In addition to cataloguing the national stock of RCFMI buildings and investigating their characteristics, the study outlined herein was designed to provide a forecast of the earthquake vulnerability of existing commercial RCFMI buildings in New Zealand in an effort to quantify the cumulative earthquake risk. Keywords Building typology · Masonry infill · Earthquake vulnerability

1 Introduction In response to the 2010/2011 Canterbury, New Zealand earthquakes, the Canterbury Earthquakes Royal Commission (hereafter referred to as the Royal Commission) was established on 11 April 2011 and given the mandate to investigate the seismic performance and sufficiency of current provisions for the design, construction, and maintenance of existing buildings in New Zealand. In its final report, the Royal Commission provided recommendations that mainly focused on advancing public awareness of earthquake-prone buildings (Canterbury Earthquakes Royal Commission 2012). The report outlined that communities

* Rijalul Fikri [email protected] 1

Department of Civil and Environmental Engineering, University of Auckland, Auckland, New Zealand

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should have proper knowledge to identify and to assess earthquake-prone buildings in an effort to reduce the potential risk posed by these buildings in future earthquakes. In New Zealand, earthquake-prone buildings are defined as those buildings that will have their maximum capacity exceeded in a moderate earthquake, making them likely to collapse and thus cause fatalities and damage (Parliament 2004). Reinforced concrete frame with masonry infill (RCFMI) buildings may be considered to be potentially earthquake-prone based on evidence from past earthquakes worldwide, including the 2004 Aceh (Indonesia) earthquake, 2009 Padang (Indonesia) earthquake, 2009 L’Aquila (Italy) earthquake, and 2015 Kathmandu (Nepal) earthquake (see Fig. 1). In most cases, the failure modes demonstrated by RCFMI buildings during earthquakes are partial or complete masonry wall failure (in-plane and/or out-of-plane), soft-storey failure, and short-column failure. RCFMI construction practices in New Zealand developed as an evolution of unreinforced masonry (URM) construction practices that were established from the early period of European settlement. Initially, URM construction reflected a colonial predisposition to emulate ‘mother country’ British construction practices, prior to the first New Zealand seismic regulations being developed in 1935 following the 1931 Hawke’s Bay earthquake (Ingham 2008). Consequently the architectural characteristic of RCFMI buildings in New Zealand share many similarities to URM buildings in other British Colonies. Among these British Colonies, New Zealand’s URM construction practices prior to the 1931 Hawke’s Bay, New Zealand earthquake were observed to be notably similar to Australian

(a)

(c)

(b)

(d)

Fig. 1 Damage to RCFMI buildings induced by earthquake shaking in various countries. a Infill wall and soft-storey failure in the 2004 Aceh (Indonesia) earthquake, b short-column failure in the 2009 Padang (Indonesia) earthquake, c infill failure in the 2009 L’Aquila (Italy) earthquake, d infill failure in the 2015 Kathmandu (Nepal) earthquake

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Author's personal copy Bulletin of Earthquake Engineering

and North American construction practices (Ingham 2008; Ingham and Griffith 2010; Russell 2010). In addition to the architectural similarities of these RCFMI buildings, economic factors and the level of earthquake engineering knowledge were comparable in these three regions (Ingham 2008; Ingham and Griffith 2010; Russell 2010). RCFMI buildings were initially constructed in New Zealand in the late 1890s and these building populations are now widespread across the country. RCFMI buildings experienced popularity from the 1920s to the 1960s because of the widespread availability of infill materials throughout New Zealand, their attractiveness for architectural wall façades, their ease of construction, and their significant cost efficiency. Figure 2 presents examples of early RCFMI construction in New Zealand. In the late 1970s, RCFMI construction experienced a significant decline following the introduction of New Zealand Seismic Code NZS 4203 in 1976 (NZS 4203 1976). This code required significant changes regarding seismic design and the construction of buildings, such as requirements for seismic detailing, ductility of reinforced concrete (RC) frames, and tying masonry infill walls to floor diaphragms (Beattie et al. 2008). Another factor leading to the significant decrease of RCFMI construction was the finding that masonry infill walls, including those composed of clay-brick and concreteblock, have a detrimental effect on a building’s overall performance during an earthquake (Kam et al. 2011). RCFMI buildings were generally constructed for commercial use, and significant numbers of these buildings can be found throughout New Zealand. According to The Building Act (2004), commercial buildings can be defined as buildings in which any natural resources, goods, services or money are either developed, sold, exchanged or stored. However, commercial buildings were not strictly used for commercial purpose alone because mixed-use buildings were also developed in a number of zones located near town centres in the regions of New Zealand (Auckland Council 2018). Mixed-use buildings combine two or more uses within a building, such as commercial premises at ground floor with multiunit residential at upper floors. Although RCFMI buildings comprise a substantial proportion of existing buildings in New Zealand, their building characteristics and seismic performance have not been thoroughly investigated at a national level. This uncertain earthquake vulnerability indicates potential risk during an earthquake event, which could endanger building occupants or

Fig. 2 Early New Zealand RCFMI buildings. Reproduced from Alexander Turnbull Library. a An RCFMI building on Queen street, Auckland in the 1910s, b Auckland Gas Company building in 1925

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nearby pedestrians. It is recognised that a significant number of studies have been previously undertaken to assess the earthquake vulnerability of RCFMI buildings in African and European countries (Cherifi et al. 2015; Costa et al. 2010; Dolce et al. 2003, 2006; Inel et al. 2008; Kappos et al. 2006; Mansour et al. 2013; Ozcebe et al. 2006; Ricci et al. 2014; Silva et al. 2014). According to these previous studies, buildings that were constructed prior to the establishment of a seismic design code in each country were forecasted to potentially have a high earthquake vulnerability, with these buildings frequently predicted to suffer substantial damage in a future large earthquake. When attempting to assess the uncertain earthquake vulnerability of RCFMI buildings in New Zealand, prior studies on the earthquake vulnerability of Australian and North American RCFMI buildings are most relevant. However, the comparative absence of published scholarly work addressing the earthquake vulnerability of Australian and North American RCFMI buildings results in the earthquake vulnerability of RCFMI construction analogous to that encountered in New Zealand remaining uncertain. Therefore, this study attempted to provide insight regarding the earthquake vulnerability of New Zealand RCFMI buildings and to address this comparative absence of critical evaluation regarding forecast seismic vulnerability of RCFMI buildings in these three regions. The past event of the 2010/2011 Canterbury, New Zealand earthquakes demonstrated that the RCFMI buildings generally performed satisfactorily as reported by Fikri et al. (2018). However, a number of mid and high-rise RCFMI buildings that were infilled by clay-brick materials were seismically vulnerable. The lesson learned from the 2010/2011 Canterbury earthquake was the architectural characteristics of RCFMI buildings, including the number of storeys and infill type materials, can affect the seismic vulnerability of these buildings during the earthquake event. Hence, a complete understanding of the architectural characteristics of these RCFMI buildings is necessary to investigate in order to forecast their earthquake vulnerability on a national level. As part of the study documented herein, an effort was made to develop an RCFMI building inventory in New Zealand by collecting individual building data in the Auckland and Dunedin regions. The collected data were then used to estimate the number of buildings nationwide. Auckland was selected as a study location because it is the largest city in New Zealand and has a high number of RCFMI buildings of different types. Dunedin was chosen because it is the second largest city on the South Island, and many of its buildings were constructed during the era when RCFMI construction was widely used. Due to the high number of buildings in Auckland, sidewalk surveys were performed in various areas having a high density of commercial buildings, including Newmarket, Mount Eden, Onehunga, Henderson, New Lynn, Manukau, Wairau Valley, and Penrose, as well as the Auckland central business district (CBD), see Putri (2015). Because there are considerably fewer commercial buildings in Dunedin than in Auckland, a sidewalk survey was conducted only in the Dunedin CBD and the buildings in this area can be considered representative of the commercial RCFMI buildings in the overall Dunedin region. For all sidewalk surveys, a total of 2905 commercial buildings were individually inspected, and 203 and 55 RCFMI buildings were documented in the Auckland and Dunedin regions, respectively. In addition, RCFMI building characteristics, including building address, infill type (clay-brick or concrete-block masonry infill), wall morphology (cavity or solid wall), geometry (building footprint and height), building continuity, and age of construction, were well documented. The collected RCFMI building data in Auckland were then cross-referenced with a data set of commercial buildings provided by the Department of Building Control and Property of Auckland Council to estimate the number and associated characteristics of the RCFMI

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building stock in the Auckland region. This data set consisted of building attributes such as: address, number of storey, year of construction, and construction system. Because the Auckland Council database was incomplete, one outcome of the reported study was to acquisition of supplementary data to augment the existing database. Based on the Auckland Council database, there are 20,441 commercial buildings with various construction systems prevalent in the region. In order to estimate the number of RCFMI buildings in the Auckland region, uninspected commercial buildings in each district were grouped into several zones which can be linked with surveyed areas. The number of RCFMI buildings was estimated using the proportion of surveyed RCFMI buildings against the total number of commercial buildings in the surveyed areas. The results indicate that there are 1831 RCFMI buildings (8.9%) out of 20,441 total commercial buildings in the Auckland region. It is noted that it was not possible to reliably estimate the total number of RCFMI buildings in the Dunedin region because there was no building dataset available for this region. The collected RCFMI building data were then categorised into four building typologies in line with infill materials and wall morphology. Generally, buildings are classified by typology for the purpose of detailed seismic vulnerability and/or risk modelling for a large-scale building assessment. However, this study was conducted by employing only initial earthquake vulnerability assessments using the Initial Evaluation Procedure (IEP) proposed by the NZSEE Study Group (2002). A detailed seismic assessment that includes the various RCFMI building typologies present in New Zealand should be performed in a future study. The study undertaken herein was exclusively focussed on the RCFMI building typology, with the intention to gain a comprehensive understanding of the likely seismic behaviour of these buildings in a future large magnitude earthquake. The forecast earthquake vulnerability obtained from the IEP was used to provisionally demonstrate the seismic vulnerability posed by these buildings, and it is recognised that a detailed seismic assessment including the more specific attributes of various RCFMI buildings typologies present in New Zealand should be performed in a future study.

2 Previous research on building typologies in various countries The term typology refers to how something is characterised according to general types. In the field of structural engineering, building typology is defined as the classification of buildings based on general attributes such as lateral-load resisting system (LLRS) and/ or materials used for building construction. The classification should be clear and welldefined to ensure uniform characterisation of the building stock in a region. The purpose of a building typology study in the context of earthquake engineering is to acquire information about building characteristics that can be utilised for seismic vulnerability assessment on a large geographical scale. The object of the study varies depending upon the purpose of the building assessment. For example, Binda et al. (2006) assessed the seismic performance of historical buildings in a seismic region in Italy. In addition, Mousavi et al. (2006) evaluated the seismic vulnerability of traditional houses on a national scale in Iran. Generally, building typologies in a country are assigned based on broad categories of LLRS and/or materials used for construction (Alam et al. 2013; Ahmad et al. 2014; Ricci et al. 2014; Mansour et al. 2013). In addition, some building typology studies provide a detailed classification of building characteristics, including the number of storeys, period of construction, and presence of masonry infill walls (Polese et al. 2008; Kappos et al.

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2006; Silva et al. 2014; Rota et al. 2008; Barbat et al. 2008; Cherifi et al. 2015; Inel et al. 2008; Costa et al. 2010). The categorisation of building typologies is dependent on the purpose of a seismic assessment in a particular country. For example, Russell and Ingham (2010) conducted a building typology study in New Zealand to assess unreinforced masonry (URM) buildings because this type of structural system is highly vulnerable to earthquakes and these buildings comprise an extensive proportion of existing buildings.

3 Determination of RCFMI building typologies in New Zealand The 203 and 55 surveyed RCFMI buildings in the Auckland and Dunedin regions, respectively, were assigned into one of four building typologies on the basis of infill materials and wall morphology, as outlined in Table 1. Each RCFMI building typology represents buildings with similar characteristics that are found throughout New Zealand.

4 Surveyed RCFMI buildings in New Zealand Figure 4 presents the distribution of the 203 and 55 surveyed RCFMI buildings in the Auckland and Dunedin regions, respectively, along with assigned typologies, and Fig. 5 shows the proportion of building typologies. Clay-brick infill commercial buildings with cavity-wall construction (Type A) were more numerous in both the Auckland and Dunedin regions when compared to solid clay-brick infill buildings (Type B). This popularity is thought to have occurred because cavity-walls proffer advantages when compared to solid walls, such as thermal insulation, fire resistance, and the requirement of less claybrick material, resulting in cost efficiency for construction. In contrast, concrete-block infill buildings with cavity walls (Type C) were less prevalent than concrete-block infill buildings with solid walls (Type D) because Type D buildings offer considerable advantages in terms of time and cost efficiency during the construction process.

Table 1 Definition of RCFMI building typologies in New Zealand Typology

Description

Type A

RC frame building infilled with clay-brick material with cavity-wall construction. The cavitywall construction generally comprises two leaves of a masonry wall separated by an air gap and connected using cavity ties to form a system (Fig. 3a)

Type B

RC frame building infilled with clay-brick material with solid-wall construction. This infill typically comprises two or more leaves of clay-bricks, with each row interlocking with no gap between the clay-bricks, to form a completely solid masonry wall (Fig. 3b) RC frame infilled with concrete-block material with cavity-wall construction. This infill typically comprises two leaves of concrete-block with dimensions of 390 mm in length, 190 mm in height, and 40 mm in thickness and these leaves are separated by an air gap. Steel rebar is generally extended out of the beam and into inner leaf of concrete-block wall (Fig. 3c) RC frame infilled with concrete-block material with solid-wall construction (Fig. 3d). Single hollow concrete-block with a standard dimension of 390 mm in length, 190 mm in height, and 190 mm in thickness are used for infill on the RC frame perimeter. Steel rebar is embedded inside hollow concrete-blocks and grouted with mortar cement

Type C

Type D

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(a)

(b)

(c)

(d)

Fig. 3 RCFMI building typologies in New Zealand. a Type A, b Type B, c Type C and d Type D

Type A Type B Type C Type D

Total RCFMI buildings: 203

(a)

Type A Type B Type C Type D

Total RCFMI buildings: 55

(b)

Fig. 4 Location of inspected RCFMI buildings by region. a Auckland and b Dunedin

5 Estimation of commercial RCFMI building stock in New Zealand According to the commercial building dataset acquired from the Auckland Council’s Department of Building Control and Property, there is a total of 20,441 commercial buildings of various construction systems in the Auckland region. Efforts were made to estimate the total number of RCFMI buildings in the Auckland region based on the identified proportion of RCFMI buildings in various surveyed areas within the region, including the Auckland CBD (Putri 2015), Newmarket, Mount Eden, Onehunga, Penrose, Henderson, New Lynn, Manukau Central, and Wairau Valley (see Fig. 6a). Buildings in these areas were individually inspected, and those with an RCFMI structural system were noted and their building characteristics were fully documented. In total, 2905 buildings (14.2%) were

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(a)

(b)

Fig. 5 Proportion of building typologies by region. a Auckland and b Dunedin CBD

individually inspected out of a total 20,441 commercial buildings. Of the 2905 inspected buildings, 203 buildings (7.0%) were noted as having an RCFMI construction system (see Table 2). In order to provide a reliable estimation of the commercial RCFMI building stock in the Auckland region, zones were defined to facilitate the estimation exercise over the large areas in the region that were not surveyed. These zones were defined as small geographical regions consisting of an assembly of contiguous areas having similar socio-economic characteristics. In total, there were 14 zones classified in the Auckland region based on the Auckland Unitary Plan provided by Auckland Council (see Fig. 6b). Commercial buildings that were not inspected were assigned to the separate zones in accordance with the building location and characteristics, and the proportion of commercial buildings having been not surveyed in each zone is presented in Table 4. The architectural characteristics of commercial buildings in the Auckland region are considered to be strongly aligned to their time period of construction, which can be

Fig. 6 The location of surveyed areas and zones. a Location of surveyed areas and b classification of zones representing the areas that were not surveyed in Auckland region

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Author's personal copy Bulletin of Earthquake Engineering Table 2 The number of surveyed commercial buildings and the proportion of RCFMI buildings in selected areas in Auckland No.

Suburbs

District

# buildings % buildings # infill % infill

1

Auckland Central Auckland City

1212

6.0

86

7.1

2 3 4 5 6 7 8 9 Total surveyed buildings Uninspected buildings Auckland region

Newmarket Mount Eden Onehunga Penrose Henderson Waitakere New Lynn Manukau Central Manukau City Wairau Valley North Shore

320 324 122 74 265 126 173 289 2905 17,536 20,441

1.6 1.6 0.6 0.4 1.3 0.6 0.9 1.4 14.2 85.8 100.0

43 27 8 6 18 15 0 0 203

13.4 8.3 6.6 8.1 6.8 11.9 0.0 0.0 7.0

categorised into four broad periods: pioneer (1840s–1880s), late Victorian and Edwardian (1890s–1910s), modern (1920s–1970s), and contemporary (post-1970s), see Gu (2010). In addition, the building characteristics in various areas were influenced by the trend of urban development in the region (Auckland Regional Council 2010). The areas that were surveyed in this study were considered to represent the characteristics from different architectural periods. For example, commercial buildings in the Auckland CBD mainly followed the architectural characteristics from the pioneer and late Victorian and Edwardian periods. In addition, many commercial buildings in Newmarket, Mount Eden, and Onehunga demonstrate architectural characteristics from the modern period, and some of these buildings exhibit contemporary architectural characteristics. Elsewhere, commercial buildings observed in North Auckland (Wairau Valley), West Auckland (Henderson and New Lynn), and South Auckland (Manukau Central) exhibit architectural features from the contemporary period, consistent with the development of these regions having predominantly commenced in the early 1970s. A forecast of the architectural characteristics of the uninspected buildings was undertaken using the available information for the dates of construction of the entire commercial building stock assigned into specific zones in the Auckland region as provided by Auckland Council. Based on the database there are 20,441 commercial buildings in Auckland, with 2905 buildings having been surveyed (and therefore their address being confirmed) and 17,536 buildings not surveyed. Of these 17,536 non-surveyed buildings, the addresses of 17,062 buildings were well-identified and these buildings were assigned into zones, while 474 buildings had incomplete records of the building addresses. It was considered that 86 buildings out of the 2905 surveyed buildings had incomplete information for the date of construction. Hence, 2819 buildings from the 2905 surveyed buildings had complete documentation for the year of construction (see Fig. 7). In addition to the details provided above, the dates of construction for 16,361 buildings of the 17,062 non-surveyed buildings were well-documented (see Fig. 9), and 760 buildings had no record for the year of construction. Consequently the total number of buildings for which both the address and the date of construction was known was

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#Bldgs. = 1,185

0.01

PDF

0.015

PDF

0.015

Auckland CBD PDF Auckland CBD

0.01 0.005 0

1850

1900

1950

Decades of Constructions

1850

1900

0.025

Onehunga PDF Onehunga

0.03

Penrose PDF Penrose

0.02

0.015

1950

2000

0.01

Henderson PDF Henderson

#Bldgs. = 254

0.01

0.005 1850

1900

1950

0

2000


1850

1900

1950

Manukau Central PDF Manukau Central

0.02

2000

(g)

0

2000

Wairau Valley PDF Wairau Valley

0.04 0.02

0.01


1950

0.06 #Bldgs. = 282

PDF

PDF

0.01

1950

1900

(f) 0.08

0.03 #Bldgs. = 166

#Bldgs. = 120

1900

1850


(e) 0.04

New Lynn PDF New Lynn

1850

0

2000


(d)

PDF

1900

(c)

PDF

PDF

PDF

0.01

0

1850


0.02 #Bldgs. = 67

0.005

0.02

0

2000

(b)

0.015

0.03

1950


0.02 #Bldgs. = 117

0

0.02 0.01

(a) 0.025

Mount Eden PDF Mount Eden

0.03 #Bldgs. = 315

#Bldgs. = 313

0.005 0

2000

0.04

Newmarket PDF Newmarket

PDF

0.02

1850

1900

1950

2000

0

1850

1900

1950

2000



(h)

(i)

Fig. 7 Normal distribution of the surveyed buildings in term of the dates of construction in different surveyed areas. a Auckland CBD, b Newmarket, c Mount Eden, d Onehunga, e Penrose, f Henderson, g New Lynn, h Manukau Central and i Wairau Valley

2819 + 16,361 = 19,180, and only these 19,180 buildings were considered in the analysis to estimate the total number of RCFMI buildings in the Auckland region. Because the dates of construction for 846 buildings (86 surveyed buildings and 760 non-surveyed buildings) out of 20,441 buildings (2905 surveyed buildings and 17,536 nonsurveyed buildings) were not available on the database, the appropriateness of the available building data was assessed to confirm the validity of employing a normal distribution methodology. The first step to complete this assessment was to plot the available information regarding the dates of construction for the surveyed buildings in different survey areas against the corresponding normal distribution, as shown in Fig. 7. Next, the accuracy associated with modelled the surveyed buildings using a suite of normal distributions was verified by performing normality tests using quantile–quantile (QQ) plots, see Fig. 8. As shown in Fig. 8, QQ plots were used to assess the expected value of the normal distribution at each quantile based on 2819 available building data. The expected values at each quantile for the 2819 surveyed RCFMI buildings was observed to be adequately

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(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

Fig. 8 Normality test using QQ plot to assess the sufficiency of surveyed buildings in different surveyed areas being modelled as normal distribution. a Auckland CBD, b Newmarket, c Mount Eden, d Onehunga, e Penrose, f Henderson, g New Lynn, h Manukau Centraland i Wairau Valley

linear and well-fitted the regression line with coefficients of determination ranging from 0.88 to 0.97, confirming that the dates of construction for the surveyed buildings were adequately assumed to be normally distributed. In a similar manner to the procedure explained above, the quality of the available information for the dates of construction for the 16,361 buildings in various zones that were not surveyed was examined, as shown in Fig. 9. The normality test was again conducted using QQ plots to assess the distribution of the available information for the dates of construction for the uninspected buildings (Fig. 10). Despite the theoretical quantiles data for some zones being less well-fitted against the regression lines, the coefficient of determination values were greater than or equal to 0.81 in all cases, confirming that the distribution of the available information for the dates of construction for the uninspected buildings was adequately assumed to be normally distributed.

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Zone 1 PDF Zone 1

Zone 2 PDF Zone 2

0.02

0.015

#Bldgs. = 611

PDF

0.01

Zone 3 PDF Zone 3

0.02

#Bldgs. = 1,559

PDF

PDF

0.02

#Bldgs. = 1,204

0.01

PDF

0.02 0.015

0.01

1950

2000

1850

1900

(a)

0.01

1850

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1950

Zone 6 PDF Zone 6

0.005

0

0

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0.015

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1850

1900

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1850

1900

1950

0

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1900

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Zone 12 PDF Zone 12 #Bldgs. = 760

0.01

(k)

1850

1900

1950

2000


(l)

0.02 0.015

PDF

0

2000

(j) #Bldgs. = 779

PDF

1950

0.005


0.01

1900

(h) 0.02

#Bldgs. = 1,961

Zone 13 PDF Zone 13

0.02

1850


Zone 11 PDF Zone 11


(i)

Zone 8 PDF Zone 8

0.01

2000

0.01

0

2000

0.005

0.005 0

1950

#Bldgs. = 538

(g) 0.015

1900

(d) 0.02

Zone 7 PDF Zone 7

0.02

#Bldgs. = 3,003

0.01

1850



Zone 10 PDF Zone 10

0.02

#Bldgs. = 383

0

2000

#Bldgs. = 467

(f)

Zone 9 PDF Zone 9

1950

0.01

0.005

2000

PDF

PDF

0.015

#Bldgs. = 866

(e) 0.015

1900

(c) 0.02

0.01


0.02

1850


PDF

PDF

PDF

0.015

#Bldgs. = 1,465

0

2000

(b) 0.02

Zone 5 PDF Zone 5

0.02

1950


PDF

1900

0

PDF

1850

0


0.03

0.01 0.005

0.005 0

Zone 4 PDF Zone 4 #Bldgs. = 1,856

Zone 14 PDF Zone 14 #Bldgs. = 909

0.01 0.005

0

1850

1900

1950

2000


(m)

0

1850

1900

1950

2000


(n)

Fig. 9 The normal distribution of the dates of construction for the commercial buildings in various zones that were not surveyed. a Zone 1, b Zone 2, c Zone 3, d Zone 4, e Zone 5, f Zone 6, g Zone 7, h Zone 8, i Zone 9, j Zone 10, k Zone 11, l Zone 12, m Zone 13 and n Zone 14

The normal distribution curves presented in Figs. 7 and 9 can be utilised to forecast the extent of the architectural characteristics that are similar between the surveyed buildings and the uninspected buildings, with the correlation between the two normal distribution curves defined based on the shape of the curves and the overlapping area between the curves. The shapes of the normal distribution curves for the surveyed building and for the uninspected buildings may be considered highly correlated when the standard deviation values between these two curves is larger than 95%. Additionally, when the overlapping area under the curves is larger than 50% it can be determined that the two normal distribution curves exhibit a high correlation (David 2001). The adopted procedure for forecasting the number of RCFMI buildings in New Zealand is explained by considering the example when estimating the number of RCFMI buildings in zone 1, which was defined as encompassing the Auckland CBD, Newmarket, Onehunga and Penrose. A total of 1728 commercial buildings with various construction systems were inspected during field surveys in zone 1, while 1238 commercial

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(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

(k)

(l)

(m)

(n)

Fig. 10 Normality test using QQ plot to assess the sufficiency of building population in the uninspected areas being modelled as normal distribution. a Zone 1, b Zone 2, c Zone 3, d Zone 4, e Zone 5, f Zone 6, g Zone 7, h Zone 8, i Zone 9, j Zone 10, k Zone 11, l Zone 12, m Zone 13 and n Zone 14

buildings were not inspected (see Table 4). The distribution for the dates of construction of these 1238 uninspected buildings in zone 1 was individually plotted for each of the surveyed areas, and it was shown that the normal curves for zone 1 had a high correlation with the normal curves for Newmarket, Mount Eden and Onehunga, see Fig. 11. The overlapping areas between the two normal distribution curves was calculated, and the extent of these overlapping areas were utilised to provide a weighted average to determine the architectural characteristics of the buildings between zone 1 and the selected surveyed areas. The number of RCFMI buildings in zone 1 was then estimated using the proportion of the surveyed areas, as presented in Table 3. The number of non-surveyed RCFMI buildings in different zones was also forecasted in accordance with the similarities of architectural characteristics, employing the same procedure as adopted for zone 1. When the entire forecasting process was completed, it was

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OVL = 91.1%

0.01

0.015

Mount Eden Zone 1 OVL Area

Onehunga Zone 1 OVL Area

OVL = 78.9%

0.01

PDF

PDF

0.01

0.015

Newmarket Zone 1 OVL Area

0.005

0.005

0.005

0 1800

1850

1900

1950

2000

OVL = 87.5%

PDF

0.015

0 1800

2050


1850

1900

1950

2000

2050


(a)

0 1800

1850

1900

1950

2000

2050


(b)

(c)

Fig. 11 Overlapping area between two normal distribution curves to forecast the architectural similarity between surveyed and uninspected buildings. a Newmarket, b Mount Eden and c Onehunga Table 3 Weighting average determined by the overlapping areas between two normal distribution curves No.

Surveyed

% similar charac- # buildings teristics

% infill

# estimated infill

1

Auckland Central

0.0

0

7.1

0

2 3 4 5 6 7 8 9

Newmarket Mount Eden Onehunga Penrose Henderson New Lynn Manukau Central Wairau Valley Total

35.4 30.7 33.9 0.0 0.0 0.0 0.0 0.0 100.0

438 379 421 0 0 0 0 0 1238

13.4 8.3 6.6 8.1 6.8 11.9 0.0 0.0

58 31 27 0 0 0 0 0 116

determined that in the Auckland region there was a total of 1831 RCFMI buildings out of 20,441 total commercial buildings, representing 8.9% of the total commercial building stock, as presented in Table 4. A recent study by Walsh et al. (2017) estimated the number of RCFMI buildings in the Auckland region at 2038 (10.2%) based on an adjustment of post-earthquake data from the Christchurch CBD that was collected by Kam et al. (2011) following the 2010/2011 Canterbury, New Zealand earthquakes. The estimated number of RCFMI buildings in the Auckland region as presented herein (1831 buildings or 8.9% of the total commercial building stock) is slightly lower than was determined in the previous study by Walsh et al. (2017), with the difference attributed to the fact that the use of post-earthquake data from the Christchurch CBD fails to fully account for the natural variability of the building stock between two different cities within New Zealand. It is proposed that the 8.9% proportion of all commercial buildings in the Auckland region that are comprised of RCFMI construction can be adopted as a nationwide estimate because it has been previously established that construction trends associated with architectural characteristics and the selection of construction systems were reasonably consistent across the entire country. For example, early European settlement in the 1840s utilised timber as the primary building material due to the outstanding quality and ample supply of timber throughout the country (Shaw and Morrison 2003). By the 1870s the supply of

13

Author's personal copy Bulletin of Earthquake Engineering Table 4 The number of commercial buildings and estimated number of buildings with an RCFMI structural system by zone No. Zones

District

Surveyed/unsurveyed

# bldgs. % bldgs. # RCFMI % RCFMI

1

Auckland city

Surveyed

1728

8.5

143

8.3

Unsurveyed Surveyed Unsurveyed Surveyed Unsurveyed Surveyed Unsurveyed Surveyed Unsurveyed Surveyed Unsurveyed Surveyed Unsurveyed Surveyed Unsurveyed Surveyed Unsurveyed Surveyed Unsurveyed Surveyed Unsurveyed Surveyed Unsurveyed Surveyed Unsurveyed Surveyed Unsurvey Unsurveyed

1238 324 1648 – 626 – 1959 289 1494 – 946 – 490 – 571 – 393 173 3054 – 2056 – 853 126 802 265 932 474 20,441

6.1 1.6 8.1 – 3.1 – 9.6 1.4 7.3 – 4.6 – 2.4 – 2.8 – 1.9 0.9 14.9 – 10.1 – 4.2 0.6 3.9 1.3 4.6 2.3 100

116 27 151 – 52 – 189 – 155 – 86 – 46 – 54 – 29 – 209 – 219 – 82 15 73 18 118 49 1831

9.4 8.3 9.2 – 8.3 – 9.7 – 10.4 – 9.1 – 9.4 – 9.5 – 7.4 – 6.8 – 10.7 – 9.6 11.9 9.1 6.8 12.7 10.3 8.9

Zone 1

2

Zone 2

3

Zone 3

4

Zone 4

5

Zone 5

6

Zone 6

7

Zone 7

8

Zone 8

9

Zone 9

10

Zone 10

11

Zone 11

12

Zone 12

Franklin

13

Zone 13

Waitakere

14

Zone 14

15 16

Unknown Auckland region

North Shore

Rodney

Manukau and Papakura

timber was declining, leading to a transition from timber construction to unreinforced claybrick masonry buildings nationwide (Hodgson 1992). However, unreinforced clay-brick masonry buildings were seismically vulnerable as was observed following the 1931 Napier, New Zealand earthquake, resulting in the subsequent restricted use of this construction system throughout New Zealand (Thornton 1996). As a result of the seismic vulnerability of unreinforced clay-brick masonry buildings, RCFMI buildings gained popularity as a construction system, with this building form primarily being adopted from the 1920s to the 1960s. It was observed that the 230 surveyed RCFMI buildings in Auckland and the 55 surveyed RCFMI buildings in Dunedin had similar architectural characteristics associated with the number of storeys, building continuity, and the infill material types (see Fig. 12).

13



Although data for the year period of construction for RCFMI buildings in the Dunedin region were not available, it is forecasted that RCFMI construction in different regions of New Zealand flourished in the period from the 1920s to the 1960s following restriction on URM construction practices nationwide. Hence, the small number of surveyed RCFMI buildings in the Auckland and Dunedin regions can be considered as being representative of the nationwide RCFMI building stock. It is recognised that the relatively small number of surveyed Auckland RCFMI buildings that were utilised to forecast the entire population of commercial RCFMI buildings in the Auckland region may lead to a bias. This bias would be reduced by surveying additional RCFMI buildings in different areas of the Auckland region and then employing this larger number of surveyed RCFMI buildings to forecast the RCFMI building population in the unsurveyed Auckland areas (zones). It is noted that side-walk surveys of RCFMI buildings across large geographical areas is time-consuming and therefore inefficient in terms of both cost and effort. Previous studies have revealed that in comparison with side-walk building surveys, the use of remote sensing technology can be an easier, faster, and more cost-efficient method to collect building data for large geographical areas (Dominici et al. 2017; Ricci et al. 2014). Although the results obtained from remote sensing occasionally exhibit some photogrammetry errors and less detailed data, the results from remote sensing with cross-referencing to the available building dataset obtained from side-walk surveys and council data can provide a viable solution in an effort to document the characteristics of a large number of RCFMI buildings. When applying the above-mentioned procedure to estimate the number of RCFMI buildings in other regions of New Zealand, the boundaries delineating zones can be determined based on the known or forecast commercial building population count and density in these regions. The number of commercial buildings can be calculated based on the address of business premises that are commonly available from district councils. However, this information may not provide data on the types of buildings associated with the business premises or any relevant information pertaining to architectural characteristics of the buildings. Because the Auckland region has the largest population of commercial buildings in New Zealand, the unsurveyed buildings in other regions of New Zealand can be assigned to larger zones than were used in the Auckland study, assuming that the unsurveyed areas encompass comparatively low building populations. An example of boundary

(a)

(b)

Fig. 12 Examples of two-storey RCFMI buildings with clay-brick infill surveyed in Auckland and Dunedin. a Auckland and b Dunedin

13


determination for unsurveyed areas is also demonstrated in Fig. 6b, where unsurveyed buildings in the Auckland City District were assigned to smaller zones when compared to the unsurveyed buildings in other districts of the Auckland region. It is noted that there is no specific requirement associated with the number of unsurveyed buildings assigned to each zone. Therefore, when determining the zone boundaries for contiguous unsurveyed areas it is important to consider any variations in socio-economic characteristics between these areas because it is assumed that trends related to development in these areas was greatly influenced by socio-economic characteristics. In addition to RCFMI buildings, it is documented that unreinforced masonry (URM) and reinforced concrete (RC) building typologies constitute a high proportion of existing buildings in New Zealand. A thorough study of the commercial URM building typology was conducted by Russell and Ingham (2010), with the nationwide proportion of commercial URM buildings having been estimated to be 6% of the total commercial building stock. Similarly, following the 2010/2011 Canterbury earthquakes Kam et al. (2011) documented the number of RC buildings in the Christchurch CBD, including commercial and residential use, with the proportion being 13% of the total building population in the Christchurch CBD. In addition, in the study reported by Walsh et al. (2017) it was estimated that 30.8% of the commercial buildings in the Auckland region were of RC construction. It is noted that the proportion of commercial buildings that are of RC construction has not been determined on a national basis and that previously reported studies have exclusively considered specific regions of New Zealand.

6 Building characteristics From the sidewalk surveys undertaken in various suburbs in the Auckland region and in the Dunedin CBD (see Fig. 4), it was observed that the masonry infill materials used in RCFMI buildings most commonly consisted of clay-brick infill as opposed to concrete-block infill, as exhibited in Fig. 13. Furthermore, the selection of infill materials was strongly associated with a building’s period of construction (see Fig. 14). RCFMI buildings were first constructed in Auckland in the late 1890s employing clay-brick as the infill material, with this construction form experiencing its greatest popularity between the 1920s and the mid1960s. RCFMI buildings with concrete-block infill were initially constructed in the 1920s, with a few such buildings constructed through to the late 1940s, the use of concrete-block as the infill material being more widely used in the early 1950s, and then concrete-block largely replacing clay-brick as the preferred infill material in the late 1960s (Isaacs 2011). Note that no Dunedin building dataset was available in a form comparable to the Auckland building dataset, and therefore the period of construction for RCFMI buildings located in Dunedin is not presented in this study. Along with masonry infill material, wall morphology (i.e., whether solid or cavity walls) was documented during the Auckland and Dunedin sidewalk surveys. Cavity walls are a type of masonry infill wall consisting of two leaves of masonry separated by an air cavity and, in some cases, interconnected by a tie system. RCFMI buildings with cavitywall construction can be identified based on the presence of air vents, weep holes, and/or a running bond pattern of the masonry infill, as presented in Fig. 15. During the sidewalk surveys, 80 and 25 RCFMI buildings with cavity-wall construction were documented in the Auckland and Dunedin regions, respectively, as shown in Fig. 13.

13



(a)

(b)

Fig. 13 Proportions of infill material by type. a Auckland and b Dunedin 35

Clay Brick Conc. Block

29

30

No. of Buildings

Fig. 14 Period of construction or date of retrofit/upgrade for surveyed RCFMI buildings in the Auckland region

25 23

25 19

20

15

15

5 0

11

10

10

6 2

15 7

6 3

2

1

3

3

2 2 1 2

4 5

Fig. 15 Example of an RCFMI building with air vents and weep holes indicating masonry-infill cavity-wall construction

To predict the effect of pounding on RCFMI buildings connected to or directly abutting other buildings, the continuity of these buildings was observed during the sidewalk survey and categorised as stand-alone isolated, row-internal, or row-end based on the position of the building with respect to adjoining buildings. Figure 16 provides photographs of the types of building continuity used in the present study. Stand-alone buildings are those that are not connected to or have a sufficient gap between adjoining buildings (Fig. 16a). Row buildings are those connected to an adjoining building or buildings. Row buildings can be

13


(a)

(b)

(c)

Fig. 16 Photographs of RCFMI building continuity types. a Stand-alone isolated building and b row-internal building and c row-end building

categorised as row-internal when the building is connected on both sides (Fig. 16b) or categorised as row-end when the building is connected on only one side (Fig. 16c). Generally, row buildings having an insufficient gap between adjacent buildings can represent significantly potential for pounding damage in an earthquake, with the effect of pounding potentially be particularly severe when the adjacent buildings have different floor diaphragm levels and storey heights (Cole et al. 2012). During the sidewalk surveys it was observed that row buildings are common in the Auckland and Dunedin regions, with proportions of 87% and 95% of the total number of surveyed RCFMI buildings, respectively (see Fig. 17). This observation is unsurprising because this type of building is often found in commercial areas where there is limited demand for alleyways to access the rear of the building. Note that buildings may have been constructed at different periods to those to which they are connected. Figure 18 presents the heights of surveyed RCFMI buildings in Auckland and Dunedin. Building height is herein differentiated as low rise (one to three storeys), mid-rise (four to six storeys), or high rise (seven or more storeys). It was observed that the majority of RCFMI buildings in Auckland and Dunedin are low-rise buildings, with proportions of 78% and 98% of the total surveyed RCFMI buildings, respectively. The high proportions of low-rise RCFMI buildings is likely because these buildings were mainly constructed from the 1920s through to the mid-1960s, when the construction of taller buildings was

(a)

(b)

Fig. 17 Building continuity types by region. a Auckland and b Dunedin

13



(a)

(b)

Fig. 18 Number of RCFMI buildings by number of storeys. a Auckland and b Dunedin

uncommon. Fewer mid-rise RCFMI buildings were documented in Auckland and Dunedin, with proportions of 18% and 2% of the total surveyed buildings in each region, respectively. In addition, 2% of the total surveyed buildings in Auckland can be classified as highrise buildings. Mid-rise and high-rise buildings were observed in the CBDs, where taller buildings are more often constructed. Table 5 exhibits the relationship between infill material, wall morphology, building continuity, and the number of storeys for the surveyed RCFMI buildings in the Auckland region. As seen in Table 5, RCFMI buildings having row continuity were most frequently documented during field surveys, with a proportion of 85% when accounting for all combinations of infill material, wall morphology, and storey height. Amongst those buildings having row continuity, RCFMI buildings with cavity clay-brick infill walls made up the highest proportion (30%), followed by RCFMI buildings with solid concrete-block (26%) infill walls and solid clay-brick (25%) infill walls. It was also observed that RCFMI buildings with concrete-block cavity walls and having row continuity were less frequently observed in the Auckland region, with a proportion of only 4% when accounting for all storey heights. The most common height for RCFMI buildings was 2-storey (39%), followed Table 5 Number of surveyed RCFMI buildings classified in accordance with different building characteristics Infill materials Morphology Continuity 1-Storey 2-Storey 3-Storey 4-Storey 5+-Storey

Total

(#) (%) (#) (%) (#) (%) (#) (%) (#) (%) (#) Clay brick

Cavity Solid

Concrete block Cavity Solid Total

13

Isolated Row Isolated Row Isolated Row Isolated Row

4

2

19 9 6 3 4 2 0 0 2 1 1 0 9 4 45 22

3

1

25 12 1 0 14 7 0 0 6 3 6 3 25 12 80 39

0

0

10 5 3 1 11 5 0 0 1 0 3 1 7 3 35 17

0

0

5 2 0 0 10 5 0 0 0 0 1 0 5 2 21 10

0

0

1 0 0 0 11 5 0 0 0 0 4 2 6 3 22 11

7

(%) 3

60 30 10 5 50 25 0 0 9 4 15 7 52 26 203 100


by 1-storey buildings with a proportion of 22% and 3-storey buildings with a proportion of 17%. Therefore, based on the sidewalk survey in Auckland it was established that low-rise (1 to 3 storey) RCFMI row buildings were the most common typologies in the Auckland region.

7 Estimated initial earthquake vulnerability of RCFMI buildings The earthquake vulnerability of surveyed RCFMI buildings in Auckland was estimated using the New Zealand Initial Evaluation Procedure (IEP) (NZSEE Study Group 2002), which provides a coarse screening method to forecast the expected earthquake performance of a building. The IEP was designed to provide an initial assessment of the seismic capacity of an existing building based on its general characteristics when compared against the standard required for new buildings in New Zealand. The IEP procedure is used by Territorial Authorities (TA) and engineering communities in New Zealand to identify existing buildings that are potentially earthquake-vulnerable and to prepare strategies for earthquake improvement of building performance. In order to assess buildings using the IEP procedure, some generic attributes of the building to be assessed need to be acquired, including the building geometry, the date of construction, and details of any seismic strengthening that was previously undertaken. The result of the seismic assessment using the IEP is expressed as a percentage of the capacity requirements for a comparable new building (hereafter referred to as percentage of New Building Standard or %NBS). Based on the %NBS, the seismic performance of the assessed buildings was grouped into one of three categories, as presented in Table 6. The surveyed RCFMI buildings in Auckland were individually assessed using the IEP procedure based on collected data. It should be noted that nine RCFMI buildings were excluded from the IEP assessment because no information was available regarding their year of construction. Similarly, it was impossible to conduct IEP assessment for the surveyed RCFMI buildings in Dunedin due to a lack of information about the year of construction. Attributes required for the IEP assessment include the seismic Hazard Factor for Auckland ( Z = 0.13 ), Seismic Zone C, Natural Period (assumed to be T ≤ 0.4 s ), Ductility Factor (assumed to be 𝜇 = 2.0 ), Near-fault Factor ( N(T, D) = 1 to account for the absence of nearby faults), and Importance Level ( IL = 2 ), following the New Zealand Standard for Structural Design Actions, Part 5: Earthquake Actions—New Zealand (see NZS 1170.5 2004).

Table 6 The classification of assessed buildings by %NBS result %NBS

Description

%NBS